Why am I not surprised to find that our old mate 411A is a 'straight down the airway centreline' (sorry, 411A, centerline) man.
...and TwoOneFour, I think you'll find that 410 is indeed saying that everyone should offset 1/2 NM so that (not in the usually one way NAT airspace, but in the rest of the world), no one will be on the centreline on a two way airway.